The present invention relates to a reflective optical modulation element unit for use in an image projection apparatus such as a projector, and to a projection optical unit having the optical modulation element unit.
A liquid crystal projector has a liquid crystal panel serving as an optical modulation element which modulates illumination light in accordance with image information input thereto and also has a projection lens which projects the modulated light onto a screen to display an image on the screen.
Japanese Patent Laid-Open No. 10(1998)-319379 has disclosed a liquid crystal projector which includes a dustproof structure for a liquid crystal panel and a cooling structure for the liquid crystal panel to improve the quality of a projected image. The projector has a cover glass provided for each of the front and the back of a transmission-type liquid crystal panel to provide a dustproof sealing space between the cover glasses and the transmission-type liquid crystal panel. Cooling wind is flowed along the cover glass surfaces to cool the transmission-type liquid crystal panel. The cover glass is disposed at a considerable distance from the liquid crystal panel to increase an amount of defocusing of a projection lens for any dust adhered to the outer surface of the cover glass to make the dust inconspicuous in the projected image.
On the other hand, each of Japanese Patent Laid-Open No. 2003-177382 and Japanese Patent Laid-Open No. 2004-219971 has disclosed a dustproof structure for a reflective liquid crystal panel used in a projector. In these disclosed structures, a dustproof cover surrounds the entire space from a beam splitter to the liquid crystal panel including optical function members such as a quarter-wave plate disposed between the beam splitter and the liquid crystal panel.
Some of the optical function members including the quarter-wave plate are made of crystal and others are of a film type. The film-type optical function members are suitable for realizing an optical system having a low F-number to illuminate the liquid crystal panel, that is, for displaying a bright image.
However, as seen in the structure disclosed in Japanese Patent Laid-Open No. 10(1998)-319379, the cover glasses for the dustproof purpose disposed at a large distance from the liquid crystal panel increase the number of parts other than the optically required parts and increase the length of the back focus from the projection lens by the distance from the liquid crystal panel to the cover glass. As a result, the overall optical system is increased in size.
On the other hand, the structures disclosed in Japanese Patent Laid-Open No. 2003-177382 and Japanese Patent Laid-Open No. 2004-219971 employ the dustproof structure in which the cover member surrounds the entire space between the beam splitter and the liquid crystal panel, so that the air within the dustproof structure cannot be circulated and thus the quarter-wave plate disposed in the dustproof structure cannot be cooled. For this reason, a heat-resistant quarter-wave plate made of crystal is used, not a quarter-wave plate of the film type susceptible to heat. However, the quarter-wave plate made of crystal leads to difficulty in realizing an illumination optical system with a low F-number.